I want to cook a lamb sholder for my family reunion this year however I do not have an oven, pit or a big bbq. So i was thinking on making a bed of coles with charcoal covering with sawdust (2-3 inches), placing down the lamb (foil wrapped of course) and then more sawdust on top 6+ inches. Covering the top. This is all kept together with bricks in a round shape to contain it all.

No sawdust but since I have been living in Italy we use a lot of olive wood chips...sounds like your idea is really good but I have my doubts about the fine sawdust you're gonna be putting under and over the beast. Even foil wrapped I think you may be getting some powder into the meat.

If I were you I'd see about wood chips of some kind which could ad flavour to the pit cooking style. It works great the way you say but NO sawdust!

I have tested the cooking method with the sawdust, and it did work. I had some condiserable doubts at the time but i was very suprised.
This what i did.

I tested with a chicken 4-5 lbs
Wrapped 3-4 times in tin foil
Set charcoal to fire and waited as long as i could.
then placed down 1-2 inches of sawdust
then chicken
then covered & packed sawdust around and on top another 2 inches
I let it cook for 3 hours then turned it carefully and let finish another hour.

I couldnt believe when it was done it was excellent and cooked to perfection.

My goal was not to add additional flavors, however it would have been great if it had. my goal was to look for an alternative cooking method.

In review, i would have allowed the charcoal to brurn until all white/grey for maximum heat. I also would have fist started the chicken above the charcoal and then placed the warmed chicken in the sawdust.

The name of this game is have patients. it does not look like it works but it is.......

Beware using sawdust from lumber as a cooking fire. A lot of the lumber used in construction or manufacture has been chemically treated and I'm not sure it's safe for preparing food. Plywood and probably your molding wood are made from many thin layers that are held together by adhesive. Wood for outdoor use is often treated with chromium or other preservatives.

I would prefer not to cook my food over burning glue and heavy metal. Maybe sawdust from untreated timber, like firewood, would be safer.